In the removal of dust from exhaust gases, e.g. of power plants, industrial processes, boilers and metallurgical and chemical plants, a variety of techniques have been used for the dry recovery of the dust or particles.
Briefly the following techniques may be mentioned: Dust removal from a gas stream by filters traversed by the gas and upon which the particulates collect, e.g. in bag or stacked filters. Such systems have the disadvantage that, as the dust collects, the pressure drop becomes more pronounced until the filters may be completely blocked. Direction-change or sedimentation-type units employ the principles that the kinetic energy of the dust particles or the momentum thereof may allow these particles to be collected against a surface upon a direction change in the gas stream, induced by baffles, or as a result of sudden decrease in the velocity of the gas stream. Such systems are effective primarily for larger particles and have poor energy efficiency.
Another technique utilizes centrifugal principles and typical of the units for separating dust by this process are cyclones and vertical-flow centrifugal separators. Such units provide means for imparting a swirl or rotary movement to the gas stream entraining the particles and centrifugally cast the particles outwardly while removing a somewhat cleaned gas from the center of the swirl pattern. Such arrangements are effective for larger particles and have only limited effectiveness for the fines which continue to be entrained by the gas when the original particle-size spectrum covers a wide range.
Finally, mention can be made of electrostatic filters which are highly effective for the removal of fines from a gas stream but may be excessively loaded by large masses of dust in the gas. In an electrostatic filter, for example, of the horizontal flow type, collector electrode plates are arranged as vertical walls defining flow passages between them, these flow passages being formed with arrays of corona-discharge electrodes which apply an electrostatic potential to the dust particles such that the latter are attracted to and collect upon the collector electrodes from which they are dislodged by rapping to allow the dust to fall into a collector bin below the electrode array.
Obviously no one dust collection technique can be effective for all purposes and to provide a minimum pressure drop, high degree of dust removal, effectiveness for large particles as well as the fines, and cost efficiency.
Thus, in many instances combinations of these techniques have been used.
For example, it has already been proposed to provide a compound separator which includes a centrifugal separator arrangement at the inlet side of horizontal flow electrostatic precipitator, the centrifugal separator having a multiplicity of dust-separating cells (see U.S. Pat. No. 2,713,920).
A dust-collecting plant of this type can be used to collect dust from industrial exhaust gases which can have a high dust content. As alluded to earlier, the centrifugal separator unit upstream of the electrostatic precipitator removes such of the coarse particle fraction, if not all of it, thereby decreasing the load on the electrostatic precipitator so as to enable the latter to operate more effectively in the removal of the fines.
A preliminary dust collection apart from the electrostatic precipitator is also advantageous if the coarse particles predominantly consist of useful or reusable materials while the fines are less usable. For instance, flue gases from a furnace which contain residual fuel dust, e.g. a furnace fired by coke particles or coal particles, generally has a particle spectrum such that the coarse particles are predominantly residual fuel while the fine particles are combustion products. In this case, the preliminary dust collection of the coarser particles allows the recovery of the fuel which can be recycled to the furnace or to the process from which the gas derives.
Such preliminary collection of a portion of the entrained particles is also advantageous where the initially collected fraction contains a significant proportion of the more abrasive particles since these abrasive particles tend to scrape fines which may have collected in the electrostatic precipitator from the electrodes thereof and reentrain these particles with the gas by reason of the sandblasting effect of such particles.
In the prior art system described and, in general where compound separators are provided, it is customary to arrange the preliminary filter or collector as a separate unit which is connected by duct work with the horizontal flow electrostatic precipitator.
To ensure an effective distribution of the incoming gas to the electrostatic precipitator, the inlet of the latter is flared from this duct outwardly toward the electrode array and baffles or the like may be provided. As a consequence, the structural steel which is required for the compound dust-collecting plant is considerable, the spatial requirements are great and the overall cost may be excessive without any significant recompense in improved efficiency.